Line Stimulation Parallel to Myofibers Enhances Regional Uniformity of Transmembrane Voltage Changes in Rabbit Hearts

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Abstract
The sign of transmembrane voltage (Vm) change (ΔVm) in the heart during unipolar point stimulation is nonuniform, which introduces dispersion of states of Vm-dependent ion channels that depends on fiber orientation. We hypothesized that line stimulation parallel to cardiac fibers increases regional uniformity of the ΔVm sign. To test this, we evaluated electrode current distribution and ΔVm produced by unipolar line stimulation in isolated rabbit hearts. The Vm-sensitive fluorescent dye, di-4-ANEPPS, and a laser scanner provided ΔVm measurements at 63 spots in an 8×8-mm epicardial region. Line stimulation was tested at specific angles with respect to the fiber direction. Current peaks occurred at electrode ends. For electrodes parallel to fibers (0°), epicardium in regions beyond the ends exhibited a nonuniform ΔVm sign, whereas epicardium between the ends exhibited a uniform ΔVm sign that was essentially negative (hyperpolarized) during anodal pulses and positive (depolarized) during cathodal pulses. The ΔVm sign between the ends became less uniform when the stimulation angle was increased relative to the long axis of the fibers. At 90°, the ΔVm sign between the ends was nonuniform and was frequently opposite, near versus away from the electrode. Spatial distributions of ΔVm during line stimulation were qualitatively predictable from anisotropic effects of point stimulation provided that combined effects of points along the electrode and points with higher current near ends were considered. For biphasic line stimulation, ΔVm during the second phase was weakly correlated with the temporal sum of effects of phases given individually, indicating limited ability of summation to predict ΔVm. Thus, uniformity of the ΔVm sign during stimulation is enhanced in the region between the ends of a line electrode parallel to fibers. This may lessen arrhythmogenic dispersion of Vm-dependent ion channel states in the region.